It seems reasonable to assume that mysql-3.23.58-1.73 was a later
revision than mysql-3.23.58-1. If this is not the case, then fine and
I obviously need a lesson in the numbering scheme.
I seem to recall that there were several changes to mysql on 7.3 and
wondered if they had been applied to the version shipped with ES.
Also, on 7.3 mysql runs as several processes - whilst on ES it is a
single process - it was this aspect that I was wondering about.

I see the confusion. On packages for supported (but not current) OS
levels, we postfix with the OS level.
mysql-3.23.58-1.73 is mysql-3.23.58-1 for RHL 7.3.
Both have been built using the same spec file with the same switches.